When two targets are associated with the same response in a speeded task, the response time is facilitated when both targets are simultaneously presented compared to when only one target is presented. This redundant-signal effect can be mediated by probability summation (race model) or by signal integration (co-activation) over and above probability summation. Previous results suggested that attending to redundant stimuli is necessary for signal integration. Here we report that probability summation also depends strongly on attention. We used stimuli that produced probability summation and manipulated attention using exogenous cueing (a flashed rectangle). A robust redundant-signal effect occurred in the attended condition where the redundant targets were presented within the cued region, but the effect was substantially reduced or absent in the ignored condition where the targets were presented in the un-cued region. Attention is thus crucial for a redundant-signal effect even for probability summation.